It has long been the experience of railroad engineers that there are situations in which it is desirable or necessary to increase the friction between the drive wheels of the locomotive and the supporting rails. Situations may arise in train starting situations or in incline braking situations or the like in which an increase in the adhesion between the drive wheels of the locomotive and the rails would enable the train to better manage the operating situation.
The tractive effort of a locomotive is the pull force it is capable of generating in order to move a train of railroad cars. The tractive adhesion of the locomotive is the tractive effort divided by the weight of the locomotive expressed as a percentage. The braking effort of a locomotive is the retarding force it is capable of generating in order to decelerate a train. The braking adhesion of a locomotive is the braking effort divided by the weight of the locomotive expressed as a percentage. The level of adhesion delivered by a locomotive is ultimately limited by the capacity of the system hardware. However, since adhesion is dependent on the frictional conditions between the steel wheel of the locomotive and the steel rail, contaminants, lubricants and other operating conditions can lower the adhesion available. A higher adhesion value is of great technical and commercial significance since it means that the locomotive can pull more freight cars or passenger cars having the same rolling resistance per vehicle.
The conventional strategy that has been pursued for many decades is to dispense sand particles ahead of some of the wheels of the locomotives which, together with wheel slip, increase the friction levels by cleaning the surfaces or by diluting the contaminants. Typical contaminants include grease, oil, water, leaves or the like that are on the rail or dropped by the train. They reduce the traction by reducing the adhesion between the drive wheels and the rails. The sand is used in the form of a powder of suitable particle size range. The sand is stored in sand boxes of locomotives and, when needed, allowed to flow by gravity feed and then blown through a hose under air pressure, out through flat nozzles and into the wheel-rail contact. The sand specification typically in use requires a minimal silica content of 90% so that the rest of the material (which, for commercially available sand, is usually clay) is not expected to be enough to significantly reduce the friction capacity of the sand.
The principal advantages of sand use are its low cost and the fact that it provides reasonable increases in adhesion under contaminated conditions. The principal disadvantages of sand are that large quantities are needed, which means significant weight added to locomotives, the damage caused to vehicles and track by sand's abrasive characteristic, and the big and somewhat expensive capital equipment facilities that are needed at railroad yards to store and dispense the sand. Further, sand simply does not provide optimal adhesion levels that are required in the railroad industry.
Apart from sand, other oxides such as alpha alumina and mullite, a mineral consisting of 3Al.sub.2 O.sub.3.multidot. 2SiO.sub.2, have also been described in the literature if not used in actual practice. See "Development of a New Method for Adhesion Improvement Replacing Traditional Sanding," K. Ohno et al, Railway Technical Research Institute, Tokyo, Japan, presented at the 1994 Miniconference of the International Heavy Haul Association, Omaha, Nebraska, Jun. 5-10, 1994. Also, U.S. Pat. No. 4,431,227 to Howell discloses a railroad wheel having cermet surface pads which are described as being capable of increasing friction between the wheel and a rail. Among the cermets which are described are oxides of aluminum and silicon, carbides of silicon and titanium, and borides of nitrogen and carbon. Industrial diamonds are also described as being useful. Similarly, U.S. Pat. No. 4,310,191 to Halldin describes railroad wheels having a peripheral wear layer made of steel or the like which had embedded in the surface carbides, oxides, borides or nitrides for increasing the frictional characteristics of the wheel. In the case of these latter two patents, the friction-enhancing material is embedded in the wheel and wears off and may not be available or suitable for specific situations which require temporary high traction or adhesion for acceleration or stopping.
It will be apparent that the prior art does not include dry powder mixtures, paste mixtures and rubbed on solid metal matrix composites, formulated to maximizing adhesion in traction and braking.